1. Field of the Invention
The invention is directed to image signal processing, and more particularly to an enhanced JPEG decoder to support decoding in a region of interest together with other capabilities.
2. Related Art
Usage of devices for displaying digital pictures has increased with the increase in the use of digital pictures. For example, information technology has advanced the transmission of information, such as speech, audio, image and video, using wireless or wired channels. This enables multimedia to be transmitted for various applications such as a personal mobile phone or the Internet. Similarly, the use of digital picture cameras has increased the usage of digital pictures. However, it is often difficult to store a large amount of multimedia information. Consequently, the multimedia information must be compressed for transmission and storage. The JPEG standard is one such compression protocol.
In the coding process of a JPEG image, an image frame is partitioned into a number of 8×8 pixel blocks referred to herein as minimum coding unit (MCU) tiles and a discrete cosine transformation (OCT) is calculated for each MCU tile to obtain OCT coefficients. The OCT coefficients are quantized, and then processed through run length coding and entropy coding to create an encoded bit-stream. One property of the OCT is an energy compaction effect which causes the transformed coefficients to be displayed from the low-frequency components at the left-upper region to the high-frequency components at the right-lower region of an 8×8 pixel block. From this property, the statistic characteristics of low-frequency or high-frequency components can be investigated to determine and generate a region of interest (ROI) during a compression process according to the bit-rate requirement.
As compared to JPEG, the JPEG 2000 protocol includes additional features such as a high compression rate, an embedded bit stream, multiple resolution representation, lossy and lossless compression, ROI, and error resilience. The ROI enhances the picture quality at the interested region during the lossy compression for transmission at a limited bandwidth. The compression standard of the JPEG 2000 protocol currently has six parts where part one builds a basic compression standard, and part two through part six are expanded from part one. In the JPEG 2000 protocol, an image frame goes through a discrete wavelet transform (DWT) and its transformed coefficients are then quantized. The transformed coefficients after quantization are partitioned into N×N-pixel codeblocks where each codeblock is processed using bit-plane coding. With a bit-plane as the coding unit, a codeblock is processed by embedded block coding with optimized truncation (EBCOT), including pass coding and arithmetic coding, bit-plane by bit-plane, to yield a high-efficiency embedded bit stream.
The coding process of JPEG 2000 comprises the three steps of:
1. Pre-processing an image frame, including tile dividing and color transforming, where the size for tile dividing is determined by the system requirement and each tile divided from an image frame is used for color transform;
2. Providing a block after the color transform for processing the DWT to remove the spatial redundancy and quantizing the transformed coefficients; and
3. Coding the transformed coefficients after quantization by bit-plane by the EBCOT to eliminate the bit redundancy and to generate an output bit stream based on the packed unit.
The JPEG 2000 in part one provides an option with ROI coding that sacrifices image quality of the uninterested region to improve image quality of the interested region (ROI). In the JPEG 2000 coding process, the ROI is first coded to yield a bit stream and has good visual quality at a limited bandwidth. Accordingly, the ROI coding is useful in applications associated with internet and wireless communications. The picture content of an image frame can be partitioned into the interested and uninterested regions in the ROI applications. The position of the ROI needs to be embedded in the coding bit stream so that the decoder can extract the ROI with good visual quality. To facilitate this, the JPEG 2000 protocol in part one includes the maxshift coding scheme to embed the information of the ROI. Therefore, the JPEG 2000, does not need additional bits to store the position of the ROI, and its decoder can effectively decode the bit stream to obtain good visual quality at the ROI.
However, processing of images using the JPEG or JPEG 2000 protocol may be slow. Decompressing an entire image in a JPEG protocol can be time consuming. Moreover when manipulating an image, such as by moving, rotating or zooming an image, a new ROI needs to be determined, thereby increasing processing. Accordingly, there is a need for improved image manipulation processing.